A mouse toe muscle was chemically skinned with saponin and the 1,0 spacing of the hexagonal myofilament lattice at a sarcomere length of 2.5 micron was measured with the X-ray-diffraction method. In the relaxed state, the 1,0 spacing was 40.8 nm. When the muscle was maximally activated at pCa 4.4, the spacing decreased to 38.4 nm. During contractions at lower calcium concentrations, the spacing decreased less. In rigor, the spacing decreased to almost the same extent as during maximum contraction, although the rigor tension was only 8% of the maximum tension. When the spacing in relaxed muscle had been adjusted osmotically to about 38 nm, activation caused no further decrease in the spacing. The results support the view that the force responsible for the lattice shrinkage during contraction is produced by cross-bridges displaced from their optimum lateral positions.